A wet electrostatic separator is a system which is installed in a conduit section of a gas flow control channel and which separates finely dispersed liquid or solid particles from a gas stream/aerosol stream. Devices of this kind are used in a broad range of fields.
The process of separating the finely dispersed particles from the gas stream includes the following steps:
electrostatic charging of the particles;
accumulating the charged particles at the surface of a collecting electrode or electrodes;
removing the charged particles from the surface of the collecting electrodes.
To electrostatically purify an aerosol, i.e. finely dispersed particles in a gas, it is customary to use negatively charged particles, i.e., ions. They are produced in a corona discharge process and form an actual electric current that flows through the air gap between an electrode that is at an electrically positive reference potential, typically ground potential, and a negative ionization electrode that is at an opposite electric potential. These electrodes are connected to a direct current-supplying high-voltage source having the requisite polarity. The value of the applied voltage is dependent on the distance between the electrodes and the properties of the gas stream to be processed.
The efficiency of an electrostatic separator is widely dependent on the intensity of the charge that is imparted by the charging section to the particles. The intensity of the charge can be enhanced by increasing the electrostatic field in the ionization section of the separator. The customary intensity maximum of the electrostatic field is limited at most to the value at which flashover begins.
In wet electrostatic separators, the ionization and collection zones are united in one system. The collection tubes are frequently long and therefore pose problems with respect to the alignment setting of the discharge electrodes. Also, the stability of the corona discharge in the ionization regions is affected by the washing/rinsing of the internal surface of the collector tubes with water. These problems are addressed in German Patents DE 101 32 582 C1 and DE 102 44 051 C1. They describe a wet electrostatic separator that includes separate ionization and collection regions. The particles are charged in an intensive electrostatic field via corona discharge processes. The corona discharge process takes place in the gap between needle or star electrodes and the openings/nozzle bores in the grounded plate when the needle or star electrodes are connected to DC high voltage. Oriented by the gas stream direction, the discharge electrodes project downstream in the gas stream into the openings/nozzle bores of the grounded plate. The charged particles are collected in the grounded tube bundle collector, which is disposed downstream in the gas stream from the high-voltage electrodes and is installed downstream in the gas stream from the ionization device.
A design of the wet electrostatic ionization stage is described in German Patent DE 101 44 051. It includes a plate which is connected to ground potential or to a positive reference/counterpotential, is mounted in a flow channel section across the inside cross section thereof, and which has a multiplicity of substantially identical openings to allow throughflow of the gas to be purified. It is followed downstream in the gas stream by a high-voltage grid, which is mounted in the channel section across the inside cross section thereof in electrical isolation therefrom, and which is connected to a high-voltage potential via a bushing in the wall of the channel section. A multiplicity of rod-shaped high-voltage electrodes corresponding in number to the openings are attached at one end to this high-voltage grid and are oriented thereto. Each of these high-voltage electrodes points toward or projects by its free end in a substantially identical manner, centrically into one opening/nozzle bore of the plate.
A disk made of or at least coated with electrically conductive material is located at each free end of such a high-voltage electrode, disposed centrally and in parallel to the plate, without contacting the same. Equally distributed over the periphery, it has at least two radial bulges/pointed tips, which are disposed radially or somewhat outwardly in a direction inclined toward the gas stream.
The operation of the wet electrostatic separator reveals that, in response to an increase in the applied voltage, i.e., in the electric field strength in the electrode gap, sparks are discharged between the electrodes and the edges of the openings/nozzle bores in correspondence with the inhomogeneous electric field. This reduces the efficiency of the particle charging and that of the particle collection in the electrostatic separator.
As shown in FIG. 5, a wet electrostatic ionization stage is made up of a multiplicity of high-voltage electrodes 1 in the form of rods which are connected by their one end to high-voltage grid 5 and have a star-shaped discharge electrode 2 mounted at the free end. Star-shaped discharge electrodes 2 are mounted axially in circular nozzle bores 3 of grounded plate 4, downstream or upstream in the gas stream from nozzle plate 4, at right angles to the direction of the gas stream. Numeral 6 denotes the nozzle bore axis.
Particle-charged gas flows through the nozzle bores. When the high voltage is applied to high-voltage grid 5, corona discharge is produced at the pointed tip locations of star-shaped electrodes 2. Gas 8 flows through the corona discharge zone; the entrained particles pick up a negative charge and exit the ionizer as negatively charged ions. It should be noted here that a positive electrical potential may, of course, also be applied to the high-voltage electrodes, and, as before, the plate may be connected to corresponding counterpotential, respectively, ground potential when the particles in the gas stream are more readily positively ionizable due to their chemical property. Finally, in certain applications, an AC high-voltage potential may also be applied to the high-voltage electrode, thereby at least entailing no technical outlay.
In an embodiment, the corona discharges may be driven at the highest possible intensity, without flashovers. As the applied voltage is increased, critical conditions are quickly reached, because the corona stream increases by approximately the square of the applied voltage. At the critical point, there is a sudden local transition from a high-field low-current-density discharge to a low-field high-current-density discharge, i.e., from a glow discharge to an arc discharge.
The entirely inhomogeneous electrostatic field between the pointed tips on star-shaped electrodes 2 and the outer end of nozzle bores 3 produces flashover discharges accompanied by decreasing efficiency of the particle charging and of the gas purification in wet electrostatic separators. The wet electrostatic ionization stage (see FIG. 5) is sensitive to the alignment setting of discharge electrodes 2 in nozzle bores 3. In the same way, the electric field of corona-discharge electrodes 2 in nozzle bores 3, which are disposed in close mutual proximity, can suppress the corona discharging at these electrodes. The result can be a decrease in the total corona stream between electrodes 2 and 3. As is discernible in FIG. 5, the corona points at the pointed tips of electrodes 2 can “see” each other, i.e., their generated fields can become mutually superposed and thereby mutually suppressed. The result is that the corona stream of the individual electrodes remains smaller than it would be if the electrode tips could not see each other.